The present invention relates to an electro-photographic image forming apparatus, and, more particularly, to a device, such as a printer and/or facsimile device, and associated method of detecting and compensating for developer deterioration therein.
Developer deterioration is defined as an improper balance of toner particles and/or carrier particles in the two-component developer necessary for creating a desired image reproduction quality in an image forming apparatus.
In the conventional image forming apparatus, a two-component developer is commonly utilized; the two-component developer includes a toner and a carrier. In this two-component developer, the non-magnetic toner particle is charged by agitating the magnetic carrier. The agitation of the two-component developer is typically performed by a developing means of the image forming apparatus. Yet, such agitation deteriorates the two-component developer over time. This deterioration in the developer degrades image quality.
Such developer deterioration may be expressed as a change in overall developer density, electrical resistance of the developer, fluidity and charge per unit of mass (Q/M). For example, it is disclosed to detect a deterioration degree of the developer by using a sensor that is detecting a quantity of the carrier component in the two-component developer (Japan Laid Open Patent No. HEI6-130818). The sensor in this arrangement measures the magnetic permeability of the two-component developer. In other words, the magnetic permeability of the two-component developer is determined by the amount of carrier occupying a predetermined volume; likewise, the sensor may also detect toner density indirectly. Yet, using a two-component developer, in an advanced deterioration state will lead to an overall density variation, such a developer density variation is independent of toner density. In other words, a magnetic permeability sensor alone cannot accurately measure the toner density when the developer has deteriorated.
In the method described above, two magnetic permeability sensors are used, one sensor is used as a toner density sensor, the other sensor is used a as a developer deterioration sensor, and both sensors are installed in different locations in the developing device. One sensor is installed in an area of low variation in overall density, used as a toner density detection sensor, while the other sensor is installed in an area of relatively higher overall density variation for use as a developer deterioration sensor. Thus, developer deterioration is determined based upon the difference of data provided by both sensors.
The limitation of the above approach is that the magnetic permeability sensor detects the developer layer on a developing sleeve as an area that the variation of density changes little overall. However, a magnetic permeability sensor can detect a wide area, thus the sensor detects an area that includes a developing sleeve and a developing roller. As the developing layer is relatively thin, the ability of the magnetic permeability sensor to accurately measure the toner density is substantially limited. It is preferred that the permeability sensor measure areas having ample developer, however, in such areas, bulk density greatly fluctuates as noted above. Thus, using sensors which measure magnetic permeability alone is inadequate.
Further, a second method is known in which the output of a toner quantity sensor and a toner density measurement are provided to detect the deterioration of a two-component developer (Japan Laid Open Patent No. HEI8-106211). This reference shows the use of an optical reflection density sensor together with a toner quality sensor or a toner quantity sensor used together with a magnetic permeability sensor. This method describes measuring the resistance value of a developer in order to determine the degree of developer deterioration. In this method, the toner density is determined by the toner density measurement sensor. Upon measuring the toner density, the degree of developer is determined by comparing the difference between a toner quantity adhered on the image carrier and an actual toner quality measured by the toner quantity sensor. Thus, the developer deterioration value depends upon the toner quantity sensor output. This method is limited in that the toner adhesion quantity, as indicated by the sensor output, is not only related to developer deterioration.
For example, the deterioration of the photo conductor is also a parameter which can impact toner adhesion quantity. When a photoconductor has deteriorated, charging ability varies so that electrostatic and image bias differ from that of an initial state. As a result, even if a developer characteristic does not vary, a toner adhesion quantity varies.
Presently, a method of detecting developer deterioration is desired in which the above-mentioned short comings are avoided.